Coupling circuit control



24, 1933. WH ER 1,931,596

- COUPLING CIRCUIT CONTROL Filed April 9, 1931 2 Sheets-Sheet l I I o OUTPUT INPUT 30 60 I20 250 500 I000 2000 4000 e000 FEE a UEN c Y (c rcLsq/szcj INPUT I 3 W v {a 7 7 0 i5 OUTPUT %Z% I 1%- b n n RESPONSE I 30 60 I20 250 500 I000 2000 4000 8000 F RE 6! uE/V C Y [trans/55c.)

INVENTOR HAROLD A. WHEELER Oct. 24, 1933. H. A. WHEELER 1,931,596

COUPLING CIRCUIT CONTROL Filed April 9, 1931 2 Sheets-Sheet 2 INVENTOR Patented Oct. 24, 1933 UNITED STATES PATENT OFFICE to Hazeltine Corporation,

Delaware a corporation of Application April 9, 1931. Serial No. 528,843 1 Claims.

This invention relates to amplifying systems,- and. more particularly to frequency-characteristic controls for vacuum tube amplifiers in audiofrequency transmission equipment.

The constant aim of engineers is the achievement of faithful reproduction of speech and music over sound-frequency transmission networks.

Due to the comparatively wide range of audible frequencies necessary for the reproduction of speech and music, (lying between 30 to 5,000 cycles per second), it is very difficult, especially at low cost, to produce an audio-frequency transmission system having a fiat response characteristic over the whole audio-frequency range. Furthermore, the acoustic properties of the surroundings where a radio receiver or loud speaker is placed may effectively diminish or accentuate certain frequencies, thereby producing the effect of distortion even though the reproducing apparatus does produce a flat response character- 1s 10.

It is a well-known fact that audio-frequency amplifiers, in general, are less efiicient at the lowfrequency extreme of their response characteris- 2 tic. The lower frequencies become relatively less audible than the higher frequencies, and their undue attenuation mars the faithful'reproduction of musical renditions where base notes play an important part. It is an object of this invention to improve the response characteristic of audio-frequency transmission equipment in such manner as to correct for its inherent deficiencies, or for the acoustic deficiencies of the loud speaker or its location, especially at the lower end of the frequency range. To this end means are provided for adjusting at will the ratio of high to lowfrequency response of the audio-frequency transmission equipment, by changing the frequency characteristics of audio-frequency coupling circuits.

A. novel feature of the invention is that it may advantageously be employed in radio receivers having one or more audio-frequency amplifying stages, in such a way as to enable the user of the radio set to adjust the audio-frequency response of the receiver. In adjusting the audio-frequency response, certain losses, or abnormal gains at certain frequencies can be emphasized or attenuated, as the case may require, and thereby the audio-frequency response of the radio set equalized to render more faithful or satisfactory reproduction. On theother hand, the application of this invention will also permit the user of apparatus, such as radio receivers or phonographs, to adjust the audio-frequency response of the receiver to accentuate such frequency components of the received music as his individual taste or liking prefers.

Another feature of this invention is that by 0 its application manufacturers of radio receivers are able, by simple and inexpensive means, to compensate for differences in tone reproduction between receivers of identical design, but mounted in diflerent types of cabinets.

Unavoidable variations incidental to mass production of radio receivers or audio-frequency transmission equipment can be compensated for by the use of this invention; and by simple adjustments of small, easily installed devices, the 0 coupling circuits of the audio-frequency transmission system can be equalized, and uniformity obtained, individually as regards frequency response, and collectively as regards acoustic performance.

This invention is also useful in radio receivers for minimizing the effects of very high audiofrequencies such as static and other interfering noises.

The invention will be more fully understood and other objects made clear from the following description read in connection with the drawings.

Referring to the drawings, Fig. 1 represents a fundamental circuit for an amplifier stage incorporating one embodiment of this invention.

Fig. 2 shows a set of curves illustrating the performance of the circuit of Fig. 1.

Fig. 3 shows a modified form of this invention incorporated in the fundamental circuit such as shown in Fig. 1.

' Fig. 4 shows a set of curves illustrating the performance of the circuit of Fig. 3.

Fig. 5 represents a typical broadcast receiver incorporating this invention.

Referring to Fig. 1, there are shown two amplifier tubes 1 and 1 coupled together by an audio- ;frequency transformer 2, having a primary winding 3 and a secondary winding 4. For the purpose of this invention, the secondary winding 4 has connected between its terminals a condenser 5 and a variable resistance 6 in series. The two elements 5 and 6 in series, shunting the secondary winding 4 of transformer 2, have a total impedance which is much greater at low frequencies than at high frequencies because of the well-known properties of the condenser.

The action of the vacuum tubes and their elements, with the necessary potential-supply sources, will not be described in detail as it is well known in the art how vacuum tube amplifiers function, and also the many conventional ers in which they are employed. This invention, being applicable to any of the wellknown types of vacuum tube amplifiers, has no direct bearing on the combined functions of the elementary circuit as shown, except in reference to the frequency response-characteristic of the system.

The shunt path, formed by condenser 5 and resistance 6 in series, serves to reduce the amplification at high audio-frequencies by a greater amount than at low audio-frequencies. In fact,-

-if condenser 5 is of sumciently great magnitude and resistance 6 has a sufiiciently low value, the addition of this shunt path may actually tune the audio-frequency transformer within the lower audio-frequency range, and thereby increase amplification over at least part of the lower audio-frequency range. Obviously, the values of condenser 5 and resistance 6 may vary within wide limits, depending upon the results desired. Representative values for these circuit elements are as follows: Condenser 5:.01 microfarad, variable resistance 6:5000 to 500,000 ohms.

These values are particularly applicable where the amplifier tubes have a plate resistance of about 10,000 ohms, and where the transformer 2 has a step-up ratio of about 1 to 3. It will be clear that in accordance with this invention, the shunt path may be connected across any suitable circuit element, such as primary coil 3, although the control is more effective when it is connected across a high-voltage element, as shown.

Fig. 2 shows curves with relative amplification or audio-frequency response, represented in arbitrary units, plotted against audio frequency, in cycles per second. Curve A indicates the normal performance of the amplifier stage in which the major part of the audio-frequency range is amplified with a fair degree of uniformity. This condition is practically unaffected when the elements 5 and 6 are added in the shunt path, and 6 is adjusted to its maximum value.

Curve B shows the effect of adjusting resistance 6 to an intermediate value. It is observed that curve B indicates only a moderate reduction in response over the lower portion of the audiofrequency range, but a somewhat greater reduction over the higher portion of the range. In other words, the ratio of low-frequency response to high-frequency response has been slightly increased. Curve C shows the response when resistance 6 is a fiusted to a low value. In this case, dueto resonance effect, the response is actually increased over the low-frequency portion, and, due to the shunt action, is greatly decreased over the high-frequency portion. In other words, the ratio of low-frequency response to high-frequency response has been greatly increased.

Fig. 3 shows another embodiment of the invention applied to an audio-frequency amplifier stage in a manner slightly different from that shown in Fig. 1. In Fig. 3, the relative low-tohigh frequency response is shown to be adjustable in several steps by means of a switch 7. When the switch is in the position shown in thefigure, the shunt path is on open circuit and has no efiect on the normal performance of the amplifier stage. As the switch is rotated to the different contacts, successively greater values of effective capacity, and successively lower values menace of resistance are inserted in series across the ter minals of the secondary coil 4 of transformer 2. In this way, the ratio of low-frequency response to high-frequency response is increased step by step and a uniform compensation of frequency= response is achieved without disproportionate increase of frequency-response in any part oi. the audio spectrum. In other words, the heterogeneous conductive characteristics of the capacitive and resistive elements are so combined that at each step such magnitudes of these elements are connected in the shunt path as to main a uniformly progressive ratio of high-frequency to low-frequency response. It is to be understood that the invention is not limited to the specific arrangement indicated in Fig. 3, as the eleents 5, 5' and 6 can be made variable in a nber of steps.

. Fig. 4 shows curves representing the m1. ance of the circuit of Fig. 3. Curve A indicates 'the audio-frequency response when the switch is in the open-circuit position. Curves B and C indicate the response as the switch '7 is moved from step A to steps B and C, respectively. In both curves B and C' it is seen that the low-frequency response is increased over a small range at the same time that the high-frequency response is decreased. It can also be observed when comparing the curves of Figure 4 with those of Figure 2 that a new result is obtained when both the capacitive and resistive elements are made simultaneously variable, for a uniformly progressive variation in the ratio of high-frequency to low-frequency response is achieved and thereby an over-emphasized response of certain 1m frequency ranges substantially avoided. Curve C of Figure 2 clearly shows the steep response peak within a narrow band of frequencies; whereas curve C of Figure 4 together with curve B, illustrates the uniformly progressive response when successive values of capacitive and resistive elements are switched in.

When the circuit of Fig. 3 is applied to an audio-frequency amplifier in which the plate impedance of the vacuum tube is about 10,000 ohms 12a and transformer 2 has a step-up ratio of about one to three, certain values for condensers 5 and 5 and resistance 6 can be suggested. For curve B the effective value of condensers 5 and 5'--in series can be .003 microfarad, and for resistance 6, 30,000 ohms. For curve C' the value of condenser 5 can be .01 microfarad, and for resistance 6, about 10,000 ohms. The values of both the condensers in series and the value of resistance may, of course, vary within wide limits depending on the particular requirements.

Fig. 5 is the circuit of a radio receiver embodying this invention. Three stages of radio-frequencyamplification, a detector, and one stage of audio-frequency amplification are shown. As 5 is now customary, the four radio-frequency coupling circuits are tuned simultaneously by mounting the four tuning condensers on a common shaft. However, the invention is. not restricted to receivers of the type illustrated, but can be-used with all systems employing an audiofrequency coupling circuit. To simplify the diagram the power supply unit is omitted therefrom, as its function in relation to a receiving system is well known in the art, wherein either batteries or alternating-current rectifying arrangements are commonly employed for the purpose. Volume control 8 increases the radio-frequency amplification, and hence the output volume, as it is rotated clockwise, dueto a double action of l increasing the shunting resistance 9. across the primary 10 of the antenna transformer 11, and simultaneously reducing the grid bias on the three screen-grid radio-frequency amplifying tubes 12, 13 and 14 by reducing the bias resistance 15 between the ground and cathodes 16, 17 and 18 of these tubes. This double volume control furnishes the necessary attenuation for satisfactory use of the receiver with very strong signals such as from powerful local stations, and, when desired, also allows the employment of the high sensitivity resulting from the use of screengrid tubes. For the sake of further simplifying the drawing, the cathode heater and filament supply connections are shown not completed. These are normally connected to suitable windings on the power transformer in accordance with well-known practice.

The invention as illustrated in Fig. 1 is employed in the system of Fig. 5 where like reference characters have the same significance. Here, as in Fig. .1, the variable resistor 6, in series with condenser 5, forms a shunt circuit across the secondary winding 4 of the input transformer 2. By ach'usting the resistor to higher or lower values, the gain-frequency characteristic of the push-pull audio amplifier may be set at the desired frequency response.

It is a well-known fact that not only in the,

audio-frequency transmission part of a radio receiver is frequency distortion of the signal likely to be found, but also distortion of the radio-frequency signal before it is fed into the audio-frequency portion; One of the most common causes of distortion occurring ahead of the audio system is the "side-band cutting of sharply tuned radio-frequency amplifying stages, wherein the higher frequency components of the signal are actually out off, By the use of the present invention, however, a partial compensation for this defect can be had by proportion ately reducing the low-frequency response of the audio amplifier. In reducing the low-frequency response the formerlyincongruous tonal reproduction, caused by the lack of high notes over the normal strength of low notes, becomes evened out and a more uniform and satisfactory reproduction obtained.

What is claimed is:

1. In a signaling system, an audio-frequency coupling device having connected betweenits terminals a shunt path which includes capacitive and resistive elements in series, said elements being so arranged as to be simultaneously variable in inverse proportion whereby the ratio of high-frequency to low-frequency responseof said system is caused to vary in uniformly propressive order. v

2. An audio-frequency amplifier including a coupling transformer having primary and secondary windings, a vacuum tube having grid electrode and cathode, a connection from one terminal of said secondary winding to the grid of said tube, a secondconnection from the other terminal of said winding to the cathode of said tube, and a shunt path between said grid and cathode formed by a plurality of fixed capacities and resistances adapted to be serially connected in progressive steps.

3. In an audio-frequency transmission system, a coupling inductance having connected between its terminals a shunt path which includes capacitive and resistive elements of different magnitudes in series, said elements being so aranged as to be successively combined in inverse proportion to their magnitudes whereby the ratio of high-frequency to low-frequency response of said system is caused to vary in uniformly progressive steps.

4. An audio-frequency amplifier including a coupling transformer having primary and secondary windings, a vacuum tube having grid electrode and cathode, a connection from one terminal of said secondary winding to the grid of said tube, a second connection from the other terminal of said winding to the cathode of said. tube, and a shunt path between said grid and cathode formed by a plurality of fixed condensers 1 and resistances of different magnitudes so arranged as to be serially connected in progressive steps and in inverse proportion to their magnitudes whereby the ratio of high-frequency to low-frequency response of said system is varied 1 in uniform progression.

'5. An audio-frequency amplifier including a coupling transformer having primary and sec ondary windings, a vacuum tube having grid electrode and cathode, a connection from one 1 terminal of said secondary winding to the grid electrode of said tube, a second connection from the other terminal of said .secondary winding to the cathode of said tube, a plurality of fixed capacities of different magnitudes in series connected to one of said terminals, and a'plurality of fixed resistances ofdifferent magnitudes in series connected to another of said terminals,

and switching means so arranged as to successively interconnect said capacities with said resistances in inverse proportion to their magnitudes whereby the ratio of high-frequency to low-frequency response of said amplifier is varied in uniformly progressive steps.

HAROLD A. WHEELER. 

